I did exactly that with my BF17! However, I used a sealed AGM size 27 and did use the top of a marine battery box and lashed it down.
I also installed a 2nd size 27 AGM in a battery box under the dinette seat on the entry door side, running the #8 wires in behind the wood trim panelling to the converter box on the other side. It's in that front closed section that isn't accessable from the outside door.
If you do a search on my name you'll see lot's of other hints for that neat Bigfoot 17!
We would like to use a group 27 battery in our Bigfoot, but the battery box is too large. So can I just remove the group 24 box and lash the 27 battery to the area where the 24 box was? I could find a plastic tray for the 27 if leakage is the concern. Since the battery is under the battery/propane "doghouse" does it have to be in a box for other reasons?